The present invention relates to a method for operating an emission control system having nitrogen oxide storage, for cleaning up a nitrogen oxide-containing, sulfur-contaminated exhaust gas from combustion equipment, especially a predominantly lean-combustion operated Otto or Diesel engine in a motor vehicle, according to the method, from time to time desulfating phases being performed for the releasing of sulfur intercalated in the nitrogen oxide storage.
As is conventional, for fuel consumption reasons, it is desirable to operate combustion equipment, such as an Otto or Diesel engine in a motor vehicle, predominantly in lean-combustion operation, i.e., having excess air in the combusted air/fuel mixture. However, in the case of Otto engines, lean-combustion operation can only be used in the range of low and medium engine load. At high engine load, it is necessary to change to at least stoichiometric operation, so as to be able to supply the desired torque. When a torque-based engine control is used, an appropriate threshold value for the so-called indicated engine torque can be used as changeover threshold, the latter being an operand formed in the engine control which is determined with the aid of a torque model involving air mass, fuel mass, and, depending on the particular application case, other variables. The indicated engine torque actually differs from engine torque available, for example, at the flywheel of an engine by friction losses not being considered.
During lean-combustion operation, a conventional three-way catalyst is not suitable for effective nitrogen oxide reduction. That is why, in emission control systems for predominantly lean-combustion operated internal combustion engines, nitrogen oxide storages, also known as nitrogen oxide storage catalysts, are installed, which intermediately store nitrogen oxides emitted during lean-combustion operation of the combustion equipment in the form of nitrate. From time to time, short-time desorption phases or regeneration phases are performed using rich mixture operation of the combustion equipment, so as to desorb the nitrogen oxides intermediately stored in nitrate form from the nitrogen oxide storage, and convert them using the available reduction media, such as uncombusted hydrocarbons and carbon monoxide. Diverse procedures suitable for this are conventional, as described, for example, in European Published Patent Application No. 0 585 900 and European Published Patent Application No. 0 598 916. The essential parameters in the nitrogen oxide regeneration of the nitrogen oxide storage are desorption duration and the exhaust gas/air ratio during the desorption. Special strategies for the selection of these desorption parameters are described, for example, in European Published Patent Application No. 0 636 770, European Published Patent Application No. 0 733 786 and German Published Patent Application No. 199 15 793.
The necessity for nitrogen oxide desorption can be detected, for example, by an NOx sensor downstream from the nitrogen oxide storage, or by a mathematical model of the nitrogen oxide storage which considers, among other things, the quantity of nitrogen oxide brought in since the last desorption, as described, for example, in European Published Patent Application No. 0 598 917, European Published Patent Application No. 0 867 604 and German Published Patent Application No. 196 35 977. Performing a nitrogen oxide desorption before changeover from lean-combustion operation to stoichiometric operation is also conventional, in order to avoid an otherwise threatening, uncontrolled release of stored nitrogen oxide, as described, for example, in German Published Patent Application No. 197 41 079.
A nitrogen oxide storage has effective NOx storability in a certain temperature window typically between 200xc2x0 C. and approximately 500xc2x0 C. If greater temperatures are reached because of increased exhaust gas temperatures, depending on the operational state of the combustion equipment, the change may be made to stoichiometric operation because of the storage effectiveness of the nitrogen oxide storage, which has become worse, but this leads to increased fuel consumption as compared to lean-combustion operation. Temperatures above approximately 800xc2x0 C. may lead to irreversible damage of the nitrogen oxide storage, and should therefore be avoided by suitable measures.
Nitrogen oxide storages are damaged, in the sense of reduced nitrogen oxide storability, by sulfur contained in the exhaust gas, which mostly traces back to sulfur-containing fuel, by the fact that, during lean-combustion operation, beside the intercalation of nitrate intercalation of sulfur, especially in the form of sulfate can occur. The intercalated sulfates are not released or decomposed, as the case may be, under the conditions of the usual nitrogen oxide desorption states, so that they increasingly lower the nitrogen oxide storability of the nitrogen oxide storage.
From time to time, desulfating phases are performed as a remedy, in which the nitrogen oxide storage is subjected to suitable desulfating conditions, by which intercalated sulfur can be released again. These desulfating conditions typically include the setting of a rich exhaust gas composition and an increased nitrogen oxide storage temperature of over 600xc2x0 C., e.g., above 650xc2x0 C., for a sufficient desulfating time which is longer than typical nitrogen oxide desorption time, as described, for example, in European Published Patent Application No. 0 869 263, European Published Patent Application No. 0 899 430, German Published Patent Application No. 197 31 624 and German Published Patent Application No. 197 47 222. The desulfating process is accompanied by a corresponding increase in fuel consumption, because of the requisite rich air ratio and the possibly required measures for heating up the nitrogen oxide storage. In addition, during the setting of the rich air ratio, the effect of a three-way catalyst, frequently used in emission control systems, which in this case, if necessary, can simultaneously function as nitrogen oxide storage, is limited, since its point of optimal functioning occurs at the stoichiometric air ratio. Besides that, after desulfating by nitrogen oxide storage heating up, its cooling down during change to an operating state having lower engine load takes longer, so that one can change only at a later time to fuel-saving lean-combustion operation.
Since the nitrogen oxide storage temperature is an important parameter for desulfating processes, it should be determined as accurately as possible. This can be done using a sensor or with the aid of a mathematical model, the latter involving a model which can be adapted by a temperature sensor, as described in German Published Patent Application No. 197 52 271.
During a desulfating process, there is the danger of a noticeable emission of sulfur compounds, particularly SO2 and H2S. A desulfating method, in which the formation and emission of undesired amounts of H2S is avoided, is described in German Published Patent Application No. 199 20 515. It is conventional that, in desulfating in the case of only a slightly rich mixture, e.g., with xcex=0.99, less hydrogen sulfide is formed than with setting a smaller air ratio of, for example, xcex=0.9. This basically makes desirable the setting of air ratios slightly below the stoichiometric value xcex=1 for the desulfating phases, the further advantage being achieved thereby that, using such air ratios at equal additional heating measures, higher nitrogen oxide storage temperatures are reached than with smaller air ratios, and the only very weak enrichment leads to a substantially negligible additional fuel consumption and an only slight deterioration of the effect of a possibly present three-way catalyst.
In full load operating conditions of the combustion equipment, particularly in the case of an internal combustion engine, usually a clearly rich air ratio of, e.g., xcex=0.9 is set, for the purpose of increasing engine torque. In this case, in a desulfating process having correspondingly high temperatures to that of the nitrogen oxide storage, there is the danger of noticeable H2S emissions, of which it may be observed that they are noticeably greater if there is already a relatively large quantity of sulfur in the nitrogen oxide storage.
To recognize, in each case, the need for desulfating, methods for the diagnosis or monitoring of NOx storing capability by the nitrogen oxide storage on the basis of measuring the storable nitrogen oxide mass or measuring the mass of the reduction medium required for nitrogen oxide desorption are described, for example, in European Published Patent Application No. 0 733 787 and German Published Patent Application No. 197 44 579. Further methods of diagnosis are based on estimating the mass of stored sulfur, as described, for example, in European Published Patent Application No. 0 869 263, European Published Patent Application No. 0 858 837 and German Published Patent Application No. 195 22 165.
In the normal, regular operation of the combustion equipment, it is advantageous to hold the nitrogen oxide storage temperature as low as possible, so as to make possible lean-combustion operation along with sufficient effectiveness of the nitrogen oxide storage, while for desulfating it is desirable to avoid severe cooling of the exhaust gas, so as to be able to reach the required increased nitrogen oxide storage temperature more easily. These requirements can be fulfilled using an emission control system having switchable exhaust gas cooling, e.g., using a two-way emission control system having an exhaust flap, by which one may switch between exhaust pipe branches of different lengths upstream of the nitrogen oxide storage, as described in German Published Patent Application No. 195 22 165 and European Published Patent Application No. 0 856 645. Other concepts for adapting exhaust gas temperature use a connectible heat transfer mechanism, as described, for example, in European Published Patent Application No. 0 905 355 and German Published Patent Application No. 196 53 958, or arrangements for varying headwind flow to the emission control system. Alternatively, one may use an emission control system optimized to severe exhaust gas cooling in connection with, for example, an electrically operated exhaust gas heating device.
In the case of application to internal combustion engines in motor vehicles, deactivating exhaust gas cooling so as to be able to reach sufficiently high desulfating temperatures is expedient in situations, among others, in which the vehicle is frequently moved under high engine load, e.g., on express highway trips and on uphill stretches. Depending on the application of the vehicle, additional combustion equipment-controlled heating measures, in this case engine-related heating measures, may be required, for example, when the vehicle is moved principally in city traffic. Such engine-related, additional heat measures, as described, for example, in German Published Patent Application No. 195 22 165 and U.S. Pat. No. 5,758,493, include a time retard of the ignition point, engine operation having different air ratios for the individual cylinders, or a rich mixture operation in connection with a secondary-air injection in the exhaust gas tract, upstream from the nitrogen oxide storage. Since they generally lead to an increase in fuel consumption, they should only be applied to a minimum degree when absolutely necessary.
It is an object of the present invention to provide a method for operating an emission control system by which the disadvantageous influences of sulfur intercalations in the nitrogen oxide storage may be held to a minimum at relatively low cost and causing the least possible interference with the normal operation of the exhaust gas-emitting combustion device.
The operating method according to the present invention includes two different desulfating operating types, namely, a main desulfating mode and a partial desulfating mode. For the main desulfating mode, the desulfating parameters are selected so that a substantially complete desulfating of the nitrogen oxide storage is achieved. The desulfating parameters for the partial desulfating mode, on the other hand, are selected so that they may be performed more frequently, but that in so doing they influence the normal operation of the combustion device less than the main desulfating process. In particular, partial desulfating requires only a low nitrogen oxide storage temperature and a shorter period of desulfating. Partial desulfating makes possible each time a partial desulfating of the nitrogen oxide storage by which, taken in total over relatively long time periods, a relatively low level of sulfur intercalated in the nitrogen oxide storage may be maintained, without main desulfating having to be performed at comparably short time intervals. The additional, targeted performing of partial desulfating besides the main desulfating process, to be performed at greater time intervals, provides the advantage that less costly and lesser fuel-using additional heating measures suffice for at least partial desulfating of the nitrogen oxide storage, so as to hold the sulfur mass intercalated in the nitrogen oxide storage relatively low over a longer time period, which avoids high H2S emission, particularly at the beginning of desulfating processes and at full load of the combustion equipment. Furthermore, the use of the two different desulfating modes makes possible an optimized application of additional heating measures controlled by combustion equipment, and optimized use of operating conditions of combustion equipment having high exhaust gas temperatures for the at least partial desulfating of the nitrogen oxide storage at minimum fuel consumption.
A process duration meter may be provided, each for the main and/or partial desulfating phases, using which, even in the case of intermittent interruptions of a desulfating process, the net duration may be automatically measured during which the relevant desulfating conditions were actually present, and thus the desulfating in question was active.
The method according to the present invention may include a suitable criterion for recognizing the necessity for performing a main desulfating phase on the basis of monitoring the nitrogen oxide storage capability of the nitrogen oxide storage and the sulfur mass brought in since the previous main desulfating process. Therefore, partial desulfating possibly performed in the meantime have no influence on the determination of the point in time for the next main desulfating process
Suitable criteria for the necessity of a partial desulfating may be provided. If an arrangement for continuous quantitative monitoring of the sulfur mass brought into the nitrogen oxide storage is present, partial desulfating may then be deemed necessary when the brought-in sulfur mass exceeds a predefinable partial desulfating threshold value which is lower than a corresponding main desulfating threshold value that indicates the need for a main desulfating process. Absent the quantitative monitoring of sulfur quantities, a predefined number of partial desulfating processes may be provided at equal time intervals between two main desulfating processes in each case, the points in time for the main desulfating processes being able to be determined from the diagnosis of the nitrogen oxide storage capability of the nitrogen oxide storage.
In the case of a main or partial desulfating process deemed necessary and requiring an additional heat measure, the additional heat measure, and thus the subsequent desulfating process, may only be activated if the nitrogen oxide storage has exceeded a certain heating-up start minimum temperature through the normal operation of the combustion equipment. This arrangement avoids activating desulfating processes at unfavorable points in time which involve a high cost of additional heat. The additional heat measure may be broken off in case the nitrogen oxide storage temperature falls below a heating-up discontinuation temperature which is lower than the heating-up start minimum temperature.
The transition from a partial to a main desulfating process may be made even if the latter is not yet necessary per se, if, due to the operating state of the combustion equipment, and thus at first without additional heat measures, the nitrogen oxide storage temperature required for this is reached, and at least a certain amount of sulfur has already been brought to the nitrogen oxide storage since the last main desulfating process. In case the temperature of the nitrogen oxide storage drops below the minimum temperature for desulfating, the appropriate additional heat measures are performed.
In a similar manner, a main or partial desulfating process may be activated, even if not yet necessary per se, in order to use an already high exhaust gas temperature, due to the combustion source operation, and this is done to perform a main or a partial desulfating process depending on the sulfur quantity brought to the nitrogen oxide storage since a last desulfating process. However, in this case the application of additional heat measures is left out of consideration, even if the temperature of the nitrogen oxide storage drops off.
To terminate a main or partial desulfating process having an additional heat measure, it may be provided that the additional heat measure be deactivated, and then that the partial desulfating process be deactivated at that point in time at which the nitrogen oxide storage temperature has fallen below a predefinable sulfur release minimum temperature. Thereby the heat brought into the nitrogen oxide storage may be used for desulfating as long as possible.
A further refinement of the method according to the present invention may avoid unfavorable lean-combustion operation of the combustion equipment after interruption of an only very short desulfating process at higher sulfur loading of the nitrogen oxide storage.
Another further development of the method according to the present invention may provide, in the application case of a motor vehicle engine having deceleration fuel cutoff, suppressing the latter during main or partial desulfating in general, or in any case at higher vehicle speeds, in order to avoid an interruption of desulfating by the lean-combustion exhaust gas composition during a deceleration fuel cutoff.
In still another further refinement of the method according to the present invention, suppression of full load operation of the combustion equipment at the beginning of a main or partial desulfating process may be provided, whereby a correspondingly strong enrichment of the air/fuel mixture is dispensed with, which precludes excessive H2S emissions during this time period.